A pulsed Electron Cyclotron Maser (E. C. M.) was developed and used to generate high power mm-waves in the W-band (75-110GHz) and the G-band (150-220GHz) frequency ranges. The relativistic electron beam (R. E. B.) was produced from a field-immersed, field-emission, cold cathode. A shaped anode cavity was designed for the optimum cavity Q, resonant frequencies, relative mode density, reflection coefficients and mode conversion in the output coupler. Two pulsed conventional field coils were used; coil#1 (maximum B-field : 9T) produced the uniform intra-cavity magnetic field and coil#2 (maximum B-field : 1T) acted as a cathode field tuning coil. The addition of the cathode tuning coil increased the useful output energy in any pulse by a factor of =400. Four diagnostics were used to determine the characteristics of the maser; 1) direct uncalibrated power monitoring, 2) calibrated frequency measurements (made using a quasi-optical diffraction grating spectrometer), 3) near field radiation pattern measurements and 4) calibrated absolute power measurements (made using a thermopile calorimeter). The following characteristics of the maser oscillation were identified: in the W-band, single mode oscillation in the TE03 mode was observed, centred at 95.2GHz, with an output power of =50kW. The cavity was crudely step-tunable with the excitation of the TE13 mode at 81.4GHz and the TE12 mode at 88.OGHz. In the G-band, multi-mode oscillation was observed at all values of the intra-cavity magnetic field. With the increased mode density at these frequencies, the maser was quasi-continuously tunable and 200GHz oscillation was observed. These results proved to be self-consistent with the device-dependent calculations used to design the system and the general E. C. M. theory developed previously.